In the electrostatic printing and plotting, or imaging, arts, various systems have been developed over the years in which a latent image first is electrostatically imparted to a moving record medium and then is developed or toned by the application of a toning material which adheres to the electrostatically produced latent image. Both dry and liquid toners have been used and each has been found to have its own peculiar characteristics, advantages and problems.
Of particular interest with regard to the present application are those systems in which liquid toners are used. U.S. Pat. No. 3,342,164 issued to Arthur M. Lewis and assigned to the assignee of this application, discloses a type of electrostatic toner head for use with liquid toner. The patentee taught the use of a head in which toner is pulled by vacuum into a developing or toning compartment bounded on one side by the record medium bearing the latent image to be developed. The partially depleted toner then left the compartment through a plurality of edge openings spaced across the moving record medium. The face of the toner head where contacted by the record medium was essentially flat; so that, to achieve an adequate seal between the toner head and the record medium, an internal pressure of at least 280 mm of mercury (11 inches) below the ambient atmospheric pressure was usually required. Maintaining such a pressure differential required the use of comparatively large, expensive pumping devices.
The clearance which must be maintained between the record medium and the counterelectrodes of the toning head in the active toning zones is usually quite small, ideally being in the order of a few thousandths of a centimeter. Since the record medium is essentially unsupported in the active toning zones, a large difference between the internal pressure of the toning zone and the ambient atmospheric pressure will limit greatly the allowable separation between the supporting members that establish the spaced relationship between the counterelectrodes and the latent image bearing side of the record medium. Other disadvantages caused by the rather large pressure differences applied in prior art toning heads are generation of higher drag forces to be overcome by the paper transport mechanism and acceleration of component wear rates to be overcome by frequent replacement or use of more expensive wear resistant materials.
Typically in liquid toned electrostatic systems, an excess of toner is applied to the record medium to ensure adequate development; then the excess is drawn away by some means and the record medium is allowed to dry. U.S. Pat. No. 3,654,659, issued to John Blumenthal, the present applicant, and also assigned to the assignee of the present application, discloses a type of liquid toner clean-off system for high speed operation. The wet record medium is drawn past an elongated suction opening which is just upstream of a narrow land lying in the same plane as the suction opening. Air is drawn over the land when suction is applied to the suction opening, thereby skimming excess toner from the record medium. While this technique has achieved a significant measure of success, the flat configuration of the clean-off head makes effective sealing with the record medium difficult, thereby requiring the use of rather high vacuum, as in the case of the toning head previously mentioned.
Those skilled in the art will appreciate that a need has existed for some time for a reliable toning head and toner clean-off head which do not rely on the use of high vacuum to ensure proper engagement between the head and the record medium. In addition, a simplified toner head geometry has been in demand. The present invention seeks to satisfy these needs.